1. Field of the Invention
The present invention relates to a method of processing a glass body such as an optical fiber preform and to a glass-processing apparatus for implementing the method.
2. Description of the Background Art
The process of producing an optical fiber preform includes many steps incorporating the heating of a glass body such as the deposition of a glass layer by using the modified chemical vapor deposition process (MCVD process), the unification of a glass rod and a glass pipe by using the rod-in collapsing process, the jointing of a glass rod to be used as the product and a holding rod, and the elongating of a glass rod and a glass pipe.
Conventionally, the heat source for heating the glass body has used a burner that burns a mixed gas of hydrogen (H2) and oxygen (O2) or a mixed gas of propane (C3H8) and O2. However, when the foregoing heat source is used, substances such as H2 and a hydroxyl group (OH group) sometimes permeate into the glass body and diffuse there. This phenomenon deteriorates the transmission loss of the optical fiber produced from the glass body.
On the other hand, in order to meet the demand for large-capacity transmission, recent optical fibers have a wide light propagating region with a complex refractive-index profile. To form such a region, it is necessary to deposit minute glass particles onto the inner surface of the starting glass pipe by applying the MCVD process for a prolonged time.
In this case, when a burner that burns a gas such as an oxyhydrogen gas is used, the prolonged heating promotes the permeation and diffusion of H2 or an OH group into the starting glass pipe. This causes the deterioration of the transmission loss. To prevent the H2 or OH group from diffusing into the light propagating region, two approaches have been employed; one is to minimize the time for depositing the minute glass particles and the other is to increase the thickness of the starting glass pipe. When the former approach is employed, the upsizing of the optical fiber preform has an undesirable upper limit. When the latter approach is employed, the thermal conduction into the starting glass pipe is impeded, reducing the rate of the formation and deposition of the minute glass particles.
To solve the foregoing problem, researchers and engineers have proposed the heating of the glass body with a thermal plasma torch, which does not use hydrogen atom. The plasma torch has a coil into which a high-frequency current is fed. The tubular main body of a torch made of, for example, silica glass is inserted into the center of the coil. When gases such as argon (Ar) and air are fed into the main body, a plasma flame can be generated according to the size of the main body. U.S. Pat. No. 5,397,372 and the corresponding Japanese patent 2818735 have disclosed a method of producing an optical fiber preform by using a plasma torch in the MCVD process. According to the disclosure, the preform can yield an optical fiber containing only small amounts of impurities such as H2 and an OH group.
Two factors affect the rate of the deposition of the minute glass particles in the MCVD process; one is the yield of forming the minute glass particles and the other is the efficiency for the minute glass particles onto the glass pipe determined by the thermophoretic effect. To increase the rate, it is important to form an optimum heating region for increasing the formation yield and deposition efficiency of the minute glass particles. In addition, it is also required to carry out a good heat processing by forming an optimum heating region while preventing the H2 and OH group from intruding into the glass body in the steps of the unification of a glass rod and a glass pipe, the jointing of a glass rod and a holding rod, and the elongating of a glass rod and a glass pipe. However, in the above-described plasma torch, the intensity of the generated plasma flame is controlled only by the collective adjustment of the flow rate of the gases and the adjustment of power of the high-frequency electric field generated by the coil.